The present invention relates to a variable displacement pump which serves as a hydraulic power source of a hydraulic device such as a power steering apparatus for vehicles.
Japanese Patent Application First Publication No. 2002-115673 discloses a variable displacement pump which is applied to a power steering apparatus for vehicles. The variable displacement pump of the conventional art includes an adapter ring fixed into a pump body, a driving shaft extending within the pump body, a cam ring swingably disposed on a fulcrum surface that is formed on an inner circumferential surface of the adapter ring, a rotor integrally formed with the driving shaft and rotatably disposed inside the cam ring, and a plurality of vanes disposed in slots that are formed on an outer periphery of the rotor in a radial direction of the rotor. The vanes are moveable to project from the slots and retreat into the slots in the radial direction of the rotor. A plurality of pump chambers are formed between the rotor, the vanes and the cam ring. Two side plates are disposed to be opposed to each other in an axial direction of the cam ring and the rotor and support the cam ring and the rotor therebetween. The pump body is formed with a suction port from which a working oil is sucked into the pump chambers and a discharge port from which the working oil in the pump chambers is discharged. First and second fluid pressure chambers are disposed between an inner circumferential surface of the adapter ring and an outer circumferential surface of the cam ring in a radially opposed relation to each other.
Further, the above-described conventional art discloses that a contour of an inner periphery of the cam ring is constituted of a shape of a suction section sucking a working fluid from the suction port, a shape of a first closed section at a bottom dead center transferring the working fluid sucked from the suction port to the discharge port after being previously compressed, a shape of a discharge section discharging the working fluid from the discharge port, and a shape of a second closed section transferring the working fluid held in the space between the adjacent vanes at a top dead center to the suction port. The portions of the inner periphery of the cam ring which corresponds to the suction section and the discharge section, respectively, are each shaped into a complete round curve and a transient curve. The portions of the inner periphery of the cam ring which corresponds to the respective closed sections are each shaped into a negative slope curve in which a radius of curvature reduces along the rotational direction of the rotor so as to always reduce a dynamic radius of the vane with respect to an increase of the rotational angle of the rotor despite the eccentric amount of the cam ring. The complete round curve and the negative slope curve are connected with each other through a high-order curve. The above-described conventional art aims to prevent a leading end of the vane from separating apart from an inner circumferential surface of the cam ring in the respective closed sections to thereby reduce a resultant pressure pulsation and generation of vibration and noise due to the pressure pulsation.